Here's what I suggest you to do:
- Open windows service and check if firebird is running.
- If it's running stop it
- Go to firebird installation folder and look for **security3.fdb** and rename it to **security3.fdb.old**
- Now copy the **security3.fdb.empty** and paste in order to clone/duplicate the file. Rename the clone to security3.fdb
- open the firebird.conf file and make sure you still have AuthServer and WireCrypt defined as  Legacy_Auth and Disabled respectively
- Open a console and go to the firebird folder: "cd C:\Program Files\Firebird\Firebird_3_0"
- Then, from there run the comand "gsec"
- Inside the gsec command type "add user SYSDBA -pw YOUR_PASSWORD" and press enter to confirm.
- Now still in the gsec command line type "quit"
- Go back to windows services and start firebird
- Try to connect to firebird